The present invention is directed to an optical layer which is predominantly of Ta.sub.2 O.sub.5, an optical element comprising such a layer, an apparatus comprising at least one XeCl excimer laser, an apparatus comprising at least one HeCd- or N.sub.2 -gas laser and on a process for producing such an optical layer.
Tantalum pentoxide as material for optical layers would have several advantages, namely
--good mechanical and optical stability, being hard and withstanding environmental stress and further forming a very dense layer; PA1 --good plasma stability in that Ta.sub.2 O.sub.5 does not change its stochiometry under plasma conditions; PA1 --extremely low scattering of light; and PA1 --withstands very high temperatures.
It is well-known that especially for high energy laser applications, absorption of energy by layers of optical elements must be kept as low as possible so that such elements withstand the high energy density.
From R. Wolf et al., "Absorption-influenced laser damage resistance of Ta.sub.2 O.sub.5 coatings", Optica Acta, 1986, vol. 33, no. 7, 919-924, it is known that (see e.g. FIG. 1) the laser damage resistance measured for light of 530 nm generally rises with falling absorption. It is especially noted that this laser damage resistance for tantalum pentoxide layers steeply rises down to an absorption value of approximately 6.5.multidot.10.sup.-3 and that below this value the laser damage resistance rises less steeply with falling absorption. The absorption of the Ta.sub.2 O.sub.5 layers, produced by resistance evaporation, is varied by varying oxygen partial pressure during layer deposition.
In the following specification absorption is specified by EQU the absorption constant k
with EQU k=n.multidot..kappa.
wherein n is the refractive index and .kappa. is the extinction coefficient (see Bergmann, Schaefer, Lehrbuch der Experimentalphysik (handbook of Experimental Physics), vol. III, Optic, edition 8, de Gruyter, p. 263 and footnote to p. 279).
Khawaja et al., Thin Solid Films, 30 (1975), 361-369, investigate the optical behaviour of sputtered tantalum pentoxide films.
From the measurements of reflectance and transmittance at varying wave-lengths of light, the curve of absorption constant k with respect to wave-length is calculated and drawn in FIG. 3 of this reference.
Thereby the authors conclude (p. 366) that such calculation of the absorption constant for wave-lengths greater than 300 nm is to be regarded as not reliable since measuring errors of reflectance and transmittance in the %o range suffice to produce errors of the absorption constant in the order of 100%.
Therefore, the authors calculate the absorption constant k by formulae (3) or (4) in Khawaja et al. Formula (3) applies for energies up to 4.51 eV which accords to 274 nm, and thus for light wave-lengths larger than 274 nm, formula (4) for energies beyond 4.51 eV and thus for wave-lengths smaller than 274 nm.
The absorption constant k so calculated is plotted in graph (d) of FIG. 7 and is, as the authors conclude, in good agreement with the experimental data.
The present inventors have calculated the absorption constant k=k.sub.1 +k.sub.2 (4) and k=k.sub.1 (3) in the respective wave-length bands above and below 274 nm according to the formulae (3) or (4) respectively of Khawaja et al. with
______________________________________ E.sub.G = 4.15eV; E.sub.g1 = 4.51eV; C.sub.2 = 6.2 and C.sub.3 = 8.2 ______________________________________
and with the values of n measured according to FIG. 7 of the cited reference. In FIG. 1 of the present application, the results are shown.
Therefrom it may be seen that for wave-lengths larger than 274 nm the absorption constant k predicted by Khawaja et al. would e.g. be: EQU 2.7.multidot.10.sup.-4 for .lambda.=300 nm, EQU 1.5.multidot.10.sup.-2 for .lambda.=308 nm, EQU 1.3.multidot.10.sup.-1 for .lambda.=325 nm.
It may further be seen that the predicted absorption coefficient k of sputtered tantalum pentoxide films would significantly rise with rising .lambda. departing from the absorption band at 298 nm according to an energy of 4.15 eV. Thus, a minimal absorption constant k would be expected at 298 nm raising to values larger than 0.01 at 308 nm.
With respect to energy to wave-length conversion, please refer to Handbook of Chemistry and Physics, CRC Press, 55th edition, 1974-1975, F-223.
From W. H. Knausenberger et al., Selected Properties of Pyrolytic Ta.sub.2 O.sub.5 Films, Journal of the Electrochemical Society, July 1973, p. 927 ff., it is known that pyrolytic Ta.sub.2 O.sub.5 shows an absorption peak at 4.20 eV energy which accords to 295 nm wave-length. In FIG. 6 of that reference the values of the absorption coefficient .alpha. as measured is plotted over photon energy. Departing from the formula ##EQU1## known e.g. from Handbook of Experimental Physics mentioned above, Knausenberger get the following results:
______________________________________ .lambda. k ______________________________________ 331 0.012 323 0.013 313 0.016 307 0.018 303 0.02 299 0.022 291 0.032 ______________________________________
They show a rising slope of absorption constant from wave-lengths well above 300 nm towards lower wave-length values. At about 308 nm, k is measured to be about 0.018. The wave-length/absorption constant course resulting from Knausenberger is shown in FIG. 2 of the present application.
From the U.S. Pat. No. 4,142,958, layers of a quarter wave-length thickness of 500-800 .ANG. are known, thus for applications for light within the infrared region. These layers are reported to present losses in the range of 0.01% for that infrared light and are made of a high index material as of tantalum pentoxide or titanium dioxide. These layers are made according to U.S. Pat. No. 4,142,958 by a reactive ion-beam sputtering process whereby titanium dioxide layers are reported to be made by sputtering a titanium dioxide target. As shown by the absorption constant courses of Khawaja and Knausenberger it is not possible from these percentage values to predict the behaviour of Ta.sub.2 O.sub.5 referenced here at wave-lengths of about 300 nm.
From F. Rainer et al., "Materials for optical coatings in the ultra-violet", Applied Optics, Vol. 24, No. 4/15 February 1985, pages 496 ff., it is known to use coating materials as ZrO.sub.2, Y.sub.2 O.sub.3, HfO.sub.2, Sc.sub.2 O.sub.3, MgO, Al.sub.2 O.sub.3, SiO.sub.2 for light in the ultra-violet region at 248 nm. Thereby the coatings which are subjected to investigation are deposited by electron beam evaporation. For light at 248 nm attenuation coefficients are reported &lt;0.001.
According to H. Demiryont et al., "Effects of oxygen content on the optical properties of tantalum oxide films deposited by ion-beam sputtering", Applied Optics, Vol. 24, No. 4/15 February 1985, pages 490 ff., it has been found that tantalum pentoxide layers have an extinction coefficient of approximately 0.02 (according to k of about 0.04) at 300 nm light dropping to about 0.01 (according to about 0.02) at 310 nm and dropping then asymptotically towards 10.sup.-3 at about 375 nm. The layers which are investigated by Demiryont are produced by ion-beam sputtering technique as e.g. described in J. M. E. Harper et al., "Technology and applications of broad-beam ion sources used in sputtering", part II, Applications, J. Vac. Sci. Technol., 21(3), September/October 1982, page 727, and in J. L. Vossen et al., "Thin film processes", Academic Press Inc., New York 1978, page 175.
The layers which were investigated by Demiryont were produced by using a target of metallic tantalum.
Nevertheless and summarizing the above, the U.S. Pat. No. 4,142,958 and the mentioned reference to Khawaja and to Knausenberger and their teaching utilized to predict tantalum pentoxide optical behaviour in the 300 nm wave-length region seem to stand against an application of such a material layer for applications where in this very light-wave range smallest possible absorption is necessitated so as e.g. and especially for high power laser applications which emit light in the said wave-length area.